Method for bonding heat sinks to overmolds and device formed thereby

ABSTRACT

A method for bonding heat sinks to packaged electronic components comprises the steps of: (a) exposing to a plasma a surface of a molded polymer formed on a substrate; (b) allowing the plasma to at least partially convert silicon-containing residue on the surface to silica; and (c) bonding an article to the surface by applying an adherent between the article and the surface. Often, the silicon-containing residue is silicone oil, a mold release compound, which may prevent the formation of a bond when using conventional bonding methods and materials. The silica layer formed on the surface of the molded polymer assists in formation of a proper bond. The plasma may be an oxygen plasma and the adherent may be selected from either a heat cured silicone-based paste adhesive with a metal oxide filler or a heat cured porous polymer film impregnated with adhesive. In particular, the film may be polytetrafluoroethylene, the adhesive may be polybutadine, and the film may be further impregnated with a metal oxide heat transfer medium, such as zinc oxide. An alternate method comprises applying the porous polymer film without plasma treatment and heat curing the film to form a proper bond.

BACKGROUND OF THE INVENTION

[0001] 1. Technical Field

[0002] This invention relates to the bonding of articles to electroniccomponent packages. More specifically, the invention relates to a methodfor bonding heat sinks to overmolds and a device formed thereby.

[0003] 2. Background Art

[0004] Electronic components, such as semi-conductor devices, are usedin increasing numbers in a wide variety of products. Generally,electronic components produce heat during operation. In some devices,the heat generated by an electronic component may build in thecomponent, or in the device, and damage the component or othercomponents in the device. Accordingly, there is often a need todissipate the heat generated by electronic components to extend the lifeof devices using such components.

[0005] Several ways of dissipating heat generated from electroniccomponents have been developed. One common method is to provide a fan inthe device to blow air through the device and to vent the heat.Unfortunately, a fan cannot always be provided in a device usingelectronic components. Another way to increase heat dissipation from anelectronic device involves increasing the surface area of the device.According to well recognized heat transfer principles, increasing thesurface area will cause an increased transfer of heat from theelectronic component to its surroundings. Unfortunately, it is generallydesirable that electronic components be as small as possible and thereis an increasing need to reduce the size of components while stillproviding sufficient heat dissipation. Yet another way to dissipate heatis to bond an electronic component to a heat sink. The purpose of a heatsink is to conduct heat away from the electronic device and thendissipate the heat from the heat sink. Heat sinks are typically madefrom a heat conductive material, such as metal, and aluminum isfrequently used since it is light weight and readily available. Suchheat sinks can easily be designed to conduct heat away from theelectrical component while simultaneously providing a large surface areafor heat dissipation. For example, the heat sink may include integralfins that provide a large surface area for a relatively small volume.

[0006] While heat sinks solve some of the problems of heat dissipation,there remains a current problem of bonding a heat sink to an electroniccomponent. For the heat generated by the component to be transferred tothe heat sink, a connection allowing heat conductance between thecomponent and the heat sink must exist. Conventionally, heat sinks aremechanically attached to electronic components and a heat conductingthermal grease is placed between the component and the heat sink, thus,providing heat conductance as needed. Unfortunately, mechanicalattachment with clips, rivets, or other devices possesses seriousdisadvantages. First, such mechanical attachments require thermalgrease, clips, and/or rivets, and other materials, increasing thematerial cost of a unit with a heat sink. Second, the mechanicalattachments increase the process costs. Third, such mechanicalattachments have proven to possess poor long term reliability. In otherwords, either the conductive path between the component and heat sink iscompromised or the mechanical attachment fails all together. Inaddition, devices such as rivets put stress on the electronic componentand may cause failure of the component during thermal expansion andcontraction.

[0007] One attempt at resolving the problem of mechanical attachmentinvolves using adhesive to create a uniform bond between an electroniccomponent and a heat sink. Unfortunately, many electronic components aremade of substances or are packaged in substances to which it is verydifficult to adhere a heat sink. For example, electronic components areoften at least partially encapsulated in polymer compounds throughinjection molding or other molding processes. Typical adhesives that areexpected to bond with polymer compounds will not bond a heat sink tosome encapsulants. This problem has been encountered in the productionof plastic ball grid array (PBGA) packages, in particular, PBGA packageswith an overmold covering electronic components mounted on the PBGA.

[0008] Thus, there existed a need to provide a method for uniformlybonding heat sinks to electronic component encapsulants.

DISCLOSURE OF INVENTION

[0009] According to the present invention, a method for bonding isprovided comprising the steps of exposing to a plasma a surface of amolded polymer formed on a substrate, allowing the plasma to at leastpartially convert silicon-containing residue on the surface to silica,and bonding an article to the surface by applying an adherent betweenthe article and the surface. By way of example, the plasma may be anoxygen plasma. Also, the molded polymer may be an overmold, thesubstrate may be an electronic component, and the article may be a heatsink. Further, the step of bonding, for example, may include heat curingthe adherent by preferentially driving heat through the article to avoidexposing the substrate to the curing temperature. One example of asuitable adherent is a silicone-based paste adhesive with a metal oxidefiller.

[0010] The present invention provides another method for bondingcomprising the steps of providing a molded polymer formed on asubstrate, wherein the molded polymer has a surface with asilicon-containing residue thereon, bonding an article to the surface byapplying a porous polymer film between the article and the surface,wherein the film is impregnated with adhesive, and heat curing the film.By way of example, the silicon-containing residue may be silicone oil,the film may be polytetrafluoroethylene, the adhesive may bepolybutadine, and the film may be further impregnated with a metal oxideheat transfer medium.

[0011] The present invention also provides an apparatus comprising amolded polymer formed on substrate, a silica layer on a surface on themolded polymer, adherent bonded to the silica layer, and an articlebonded to the adherent. The silica layer thus provides a surface on themolded polymer to which an adherent may be adequately bonded. Anotherapparatus comprises a molded polymer formed on a substrate,silicon-containing residue on a surface of the molded polymer, anadherent bonded to the surface, and an article bonded to the adherent.By way of example, the adherent may be a porous polymer film impregnatedwith the adhesive as described above. In each of the above two methodsand two apparatus, the molded polymer may, for example, be an overmold,the substrate may be an electronic component, and the article may be aheat sink.

[0012] The foregoing and other features and advantages of the presentinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawing.

BRIEF DESCRIPTION OF DRAWINGS

[0013] Preferred embodiments of the present invention will hereinafterbe described in conjunction with the appended drawings, where likedesignations denote like elements, and:

[0014]FIG. 1 is a cross-sectional view of an overmolded plastic ballgrid array package with a heat sink bonded thereto according to apreferred embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0015] According to a preferred embodiment of the present invention, amethod for bonding is provided comprising the steps of: (a) exposing toa plasma a surface of a molded polymer formed on a substrate; (b)allowing the plasma to at least partially convert silicon-containingresidue on the surface to silica; and (c) bonding an article to thesurface by applying an adherent between the article and the surface.Preferably, the silicon-containing residue is silicone oil and theplasma is an oxygen plasma. Also, preferably the adherent is selectedfrom either a heat cured silicone-based paste adhesive with a metaloxide filler or a heat cured porous polymer film impregnated withadhesive. More preferably, the film is polytetrafluoroethylene (PTFE),the adhesive is polybutadine, and the film is further impregnated with ametal oxide heat transfer medium.

[0016] Also, a preferred embodiment of the present invention includes amethod for bonding wherein the heat cured porous polymer film describedabove is used as the adherent without plasma treatment of the moldedpolymer surface. Thus, the molded polymer has a surface withsilicon-containing residue thereon, but a bond between an article andthe surface is still established by the film.

[0017] Also according to the preferred embodiment of the presentinvention, various apparatus with a unique structure will be produced bythe methods described above. Specifically, one preferred embodiment isan apparatus comprising a molded polymer formed on a substrate, a silicalayer on a surface of the molded polymer, adherent bonded to the silicalayer, and an article bonded to the adherent. Such an apparatus willresult from a method including plasma treatment of thesilicon-containing residue on the polymer surface. Also, a preferredembodiment of present invention provides an apparatus comprising amolded polymer formed on a substrate, silicon-containing residue on asurface of the molded polymer, adherent bonded to the surface, and anarticle bonded to the adherent. Such is the type of apparatus thatresults when a method described above is used that does not includeplasma treatment of the molded polymer surface.

[0018] The need for the present invention arises from difficulty inuniformly bonding heat sinks to the encapsulants commonly used topackage electronic components. To encourage the release of encapsulatedcomponents from a mold, silicone oil or other mold release compounds arefrequently included in the polymer encapsulant. Since such mold releasecompounds encourage release from the mold, they also often prevent asecure adhesive bond between the encapsulant and a heat sink. Theproblem is that the surface to be bonded to a heat sink must either besilicone free or the adhesive must be able to nevertheless bond with theencapsulant. This problem has previously been encountered in theproduction of plastic ball grid array (PBGA) packages, in particular,PBGA packages with an overmold covering electronic components mounted onthe PBGA. Essentially, the silicone oil in the overmold prevented theadherence of a room temperature curing two part adhesive that wasintended to bond a heat sink to the overmold. Accordingly, the preferredembodiments summarized above either provide a silicon-free bondingsurface or create a bond despite the presence of silicone oil. Thepreferred embodiments are discussed in more detail below.

[0019] Referring to FIG. 1, a plastic ball grid array (PBGA) package 100is shown. PBGA package 100 includes a semiconductor device 30mechanically connected to a laminate 10 by a laminate adherent 40 andelectrically connected to laminate 10 by a device interconnect 20.Laminate 10 additionally includes a ball grid 50 used to connect PBGApackage 100 electrically and/or mechanically to other devices, forexample, to a printed circuit board (not shown). Also shown in FIG. 1 isovermold 60, a molded polymer formed over semiconductor device 30 anddevice interconnect 20 by transfer molding. A heat sink 90 is in turnmechanically connected to overmold 60 by overmold adherent 80 placed onovermold surface 70. Provided that overmold adherent 80 is at leastpartially heat conductive, rather than heat insulative, heat generatedby semiconductor device 30 will be conducted to heat sink 90 anddissipated to the surroundings of PBGA package 100. Even though FIG. 1shows PBGA package 100 with heat sink 90 bonded thereto by overmoldadherent 80, it should be noted that the present invention is applicableto other packages, as well as articles that may need bonding to suchother packages. In particular, the present invention is applicable tobonding any article to the surface of a molded polymer, wherein asilicon-containing residue on the surface of the molded polymer preventsforming a heat conductive, uniform bond between the article and thesurface.

[0020] Accordingly, a preferred embodiment of the present inventionprovides a method whereby the interference of silicone oil or othersilicon-containing residues may be overcome in forming a heat conductivebond between a molded polymer and an article, for example, a heat sink.Once the package is formed, such as PBGA package 100, the first step isto expose overmold surface 70, or another bonding surface, to a plasma.Next, the plasma conditions are maintained for a sufficiently long timeto convert to silica any silicone oil or other silicon-containingresidue on the surface that will interfere with bonding. Preferably, theplasma is an oxygen plasma and more preferably, the oxygen plasma ismaintained for abbout 3 minutes in a reactive ion etch (RIE) chamber, orequivalent. A suitable plasma will be produced by establishing apressure of about 0.35 torr in the RIE chamber with an oxygen flow rateof about 75 standard cubic centimeters/minute (sccm) while deliveringabout 300 watts of generator power. The type of plasma and plasmaconditions selected may vary depending upon the particularsilicon-containing residue and the type of polymer used for the moldedpolymer, as well other factors. However, the proper conditions can bequickly determined through trial plasma exposures followed by visualand/or chemical analysis of the treated surface by scanning electronmicroscope (SEM). By comparing the surface topography prior to treatmentand after treatment, one of ordinary skill in the art can determine whena topography change has occurred indicating conversion of the surfaceresidue to silica. Silica is a glass derivative that will exhibit adifferent texture or topography compared to a polymer material used in atypical electrical component encapsulant. Specifically, the topographychanges from a surface that appears relatively smooth at a magnificationof about 5000× to a surface that appears much more rough in texture.Once the needed conversion has taken place, bonding of an article to thesurface may occur by applying an adherent between the article and thesurface. Depending upon the type of adherent selected, additionaltemperature or pressure may be required to complete the bonding step.

[0021] For example, the types of adherents that are best for bondingheat sinks to electronic component packages require heat curing.According to a preferred embodiment of the present invention, such heatcuring occurs by preferentially driving heat through the article, forexample, heat sink 90, to avoid exposing the electrical component orother substrate within the molded polymer to the temperature needed forcuring. Because heat sink 90 is made from a heat conductive material,heat applied exclusively to the heat sink 90 will be absorbed thereinand conducted to the adherent, such as laminate adherent 40 shown inFIG. 1, thus curing the adherent. By preferentially driving heat throughheat sink 90, heat exposure of semiconductor device 30 can be diminishedbelow the point wherein permanent damage may occur.

[0022] One type of adherent according to a preferred embodiment of thepresent invention is a silicone-based paste adhesive with a metal oxidefiller. Such a silicone-based adherent is preferred because it isbelieved that the silicone in the adhesive interacts with the silicaformed on the overmold surface 70, or other treated surface, to producea stronger bond between heat sink 90 and overmold surface 70. The metaloxide filler or, more preferably, alumina filler improves the heatconductivity of overmold adherent 80, such that heat is adequatelyconveyed from semiconductor device 30 to heat sink 90. One suitableadhesive is 1-4373, available from Dow Corning in Corning, N.Y., whichis preferably cured at a temperature of about 130 to 160 degrees Celsius(° C.), or more preferably 150° C., for about 30 to 80 minutes (min), ormore preferably 60 min, without applying any pressure. Prior to curing,an initial placement pressure of about 1 to 3 pounds per square inch(psi) is applied for about 1 to 60 seconds (sec), or more preferably 10sec, to squeeze out excess adhesive between overmold surface 70 andheatsink 90. Other adhesives known to those skilled in the art may alsobe in keeping with the above criteria, but may have different curingconditions.

[0023] For some applications, it may be advisable to bake out moisturefrom overmold 60 before adhering heat sink 90, otherwise, moisture mayescape from overmold 60 while curing and form steam pockets in overmoldadherent 80. The moisture bake out typically occurs at a temperaturebelow the curing temperature for a period of several minutes, dependingon the particular material from which overmold 60, or anotherencapsulant, is formed. For example, moisture will be suitably baked outat 125° C. applied for about 1 hour. Even though it may be advisable insome applications to eliminate steam pockets in overmold adherent 80, itmay be more advisable to avoid exposing semiconductor device 30 to heat.Some heat exposure will generally be required to cure overmold adherent80, however, it is often advisable to minimize heat exposure byeliminating steps such as a moisture bake out if possible.

[0024] Alternative to the plasma surface treatment described above,another preferred embodiment of the present invention provides a methodfor bonding wherein no plasma treatment is required. Such a preferredmethod begins with providing a molded polymer formed on substrate,wherein the molded polymer has a surface with silicon-containing residuethereon. Exemplary molded polymers include overmold 60 shown in FIG. 1and common silicon-containing residues include silicone oil typicallyused to produce overmold 60. Next, bonding of an article to the surface,such as overmold surface 70, occurs by applying a porous polymer filmbetween the article and the surface, wherein the film is impregnatedwith adhesive. Any adhesive known to those skilled in the art may beused that is consistent with the features and advantages of theinvention described herein. Formation of the bond is completed with thestep of heat curing the film and, preferably, applying pressure duringthe heat curing. Typically, a film adhesive could not be used to bondheat sink 90 to overmold surface 70 because air becomes trapped betweenthe film adhesive and overmold 70 or heat sink 90 during application ofthe film. The air pockets create multiple problems in such an electroniccomponent package. First, the air pockets compromise the heat conductingpath between semiconductor device 30 and heat sink 90 since air is apoor heat conductor. In addition, the presence of the air pocketsprevents formation of a uniform bond between overmold surface 70 andheat sink 90. Thus, the strength of the mechanical attachment is alsocompromised and failure of the attachment between heat sink 90 andovermold surface 70 is likely to be accelerated.

[0025] According to a preferred embodiment of the present inventionparticular components for a porous polymer film have been developed thatform a more preferred bond with a molded polymer having silicone oilresidue on its surface. The more preferred film is made frompolytetrafluoroethylene (PTFE) impregnated with polybutadine and a metaloxide heat transfer medium. Most preferably, the heat transfer medium iszincoxide. Testing has shown that such a PTFE film can be bonded at 140to 160° C. by applying a pressure of 50 to 500 psi in a bonding cycle ofless than 10 sec for a tack cure, followed by a full cure at the sametemperature and pressure for about 10 to 60 min. Notably, a tack cure isnot required in all applications, but is an optional step to positionheatsink 90 prior to full cure. When heat sink 90 is made from anodizedor chromated aluminum the bond thus formed will exhibit greater than 300psi lap sheer strength and may go as high as 800 psi. In fact, when highbonding conditions are used, that is, pressure between 500 to 1,000 psiand a bonding cycle of greater of than ten minutes, the bond strengthbetween heat sink 90 and overmold surface 70 is stronger than the bondstrength between overmold 60 and laminate 10. Accordingly, heat sink 90will remain attached in conditions so extreme as to cause overmold 60 tobreak free from laminate 10. Even though a method including a porouspolymer film as adherent 70 does not require plasma treatment ofovermold surface 70, plasma treatment may nevertheless occur prior toapplying the film between heat sink 90 and overmold surface 70. Thetreatment steps are essentially the same as those described above exceptit is possible that lower pressure and shorter bonding time may be usedto complete the bond.

[0026] The film used in a preferred embodiment of the present inventionis advantageous over conventional films in that it is porous and thusprovides lateral dispersion of any air pockets that develop between heatsink 90 and the film or between the film and overmold surface 70 duringapplication of the film between heat sink 90 and overmold surface 70.Because any trapped air pockets in overmold adherent 80 may laterallydisperse during processing, a uniform bond will be formed and thepotential heat conductance of overmold adherent 80 will not becompromised. In addition, the mechanical strength of the bond willachieve its maximum potential since few, if any, air pockets will bepresent to compromise the mechanical strength. The most preferred formof PTFE film to provide this characteristic is essentially a film that,on the microscopic level, has the appearance of a sponge, wherein thewebs in such micro sponge are about one micrometer in diameter orslightly larger. A suitable porous polymer film is available from W. L.Gore & Associates in Newark, Del. called Gore No. 1308 and isimpregnated with approximately ten to fifty weight percent polybutadine.Even though it is not necessary to plasma treat overmold surface 70 whenusing porous polymer film as described above, plasma treatment maynevertheless be applied prior to applying the film between heat sink 90and overmold surface 70. Accordingly, there are two preferred adherentsthat may used according to the present invention following plasmatreatment to convert silicon-containing residues to silica.Alternatively, another overmold adherent 80 may also be used that is inkeeping with the criteria described above.

[0027] When the described methods are used, unique apparatus accordingto a preferred embodiment of the present invention will be producedthereby. When the plasma treatment embodiment is used, a silica layerwill exist on overmold surface 70. Such a silica layer helps provide anadequate surface for bonding of overmold adherent 80. In particular,when overmold adherent 80 comprises a silicone-based paste adhesive itis believed that the silicone in the adhesive interacts with the silicalayer on overmold surface 70 to improve bond strength.

[0028] Nevertheless, when plasma treatment is not used, and instead aporous polymer film as described above is used, a unique structure isalso present. In particular, a silicon containing residue exists onovermold surface 70, and yet the porous polymer film is securely bondedto overmold 60. In conventional packages, the presence ofsilicon-containing residue, especially silicone oil, generally precludesexistence of a bond between heat sink 90 and overmold surface 70, exceptin the case wherein mechanical devices, such as clips and rivets, areused to attach heat sink 90. Even if adhesives could be developed forfilms that will bond to such a silicon-containing surface, air pocketswould exist between the surfaces and compromise bond strength and heatconductivity. However, because the more preferred film is porous andprovides lateral dispersion of air pockets, an adequately strong andheat conductive bond may be formed. The heat conductivity is furtherenhanced by the presence of a zinc oxide heat transfer mediumimpregnated in the film.

[0029] While the invention has been particularly shown and describedwith reference to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention. Accordingly, unless otherwise specified, any dimensions ofthe apparatus indicated in the drawings or herein are given as anexample of possible dimensions and not as a limitation. Similarly,unless otherwise specified, any sequence of steps of the methodindicated in the drawings or herein are given as an example of apossible sequence and not as a limitation. For example, it will beunderstood that, while various of the conductors (connections) are shownin the drawing as single lines, they are not so shown in a limitingsense, and may comprise plural conductor (connections), as is understoodin the art.

We claim:
 1. A method for bonding comprising the steps of: exposing to aplasma a surface of a molded polymer formed on a substrate; allowing theplasma to at least partially convert silicon-containing residue on thesurface to silica; and bonding an article to the surface by applying anadherent between the article and the surface.
 2. The method of claim 1,wherein the molded polymer comprises an overmold, the substratecomprises an electronic component, and the article comprises a heatsink.
 3. The method of claim 1, wherein the silicon-containing residuecomprises silicone oil.
 4. The method of claim 1, wherein the plasmacomprises oxygen plasma.
 5. The method of claim 4, wherein the oxygenplasma is maintained for about 3 minutes at a pressure of about 0.35torr with an oxygen flow rate of about 75 standard cubiccentimeters/minute while delivering about 300 watts of generator power.6. The method of claim 1, wherein the step of bonding the articlefurther comprises heat curing the adherent.
 7. The method of claim 6,wherein the heat curing comprises preferentially driving heat throughthe article to avoid exposing the substrate to the temperature neededfor curing.
 8. The method of claim 1, wherein the adherent comprises asilicone-based paste adhesive with a metal oxide filler.
 9. The methodof claim 1, wherein the adherent comprises a porous polymer filmimpregnated with adhesive.
 10. The method of claim 9, wherein the filmcomprises polytetrafluoroethylene, the adhesive comprises polybutadine,and the film is further impregnated with a metal oxide heat transfermedium.
 11. A method for bonding comprising the steps of: providing amolded polymer formed on a substrate, wherein the molded polymer has asurface with silicon-containing residue thereon; bonding an article tothe surface by applying a porous polymer film between the article andthe surface, wherein the film is impregnated with adhesive; and heatcuring the film.
 12. The method of claim 11, wherein the molded polymercomprises an overmold, the substrate comprises an electronic component,and the article comprises a heat sink.
 13. The method of claim 11,wherein silicon-containing residue comprises silicone oil, the filmcomprises polytetrafluoroethylene, the adhesive comprises polybutadine,and the film is further impregnated with a metal oxide heat transfermedium.
 14. The method of claim 11, further comprising the steps of:exposing the surface to an oxygen plasma; and allowing the oxygen plasmato at least partially convert the silicon-containing residue to silica.15. The method of claim 11, wherein the step of heat curing comprisespreferentially driving heat through the article to avoid exposing thesubstrate to the temperature needed for curing.
 16. An apparatuscomprising: a molded polymer formed on a substrate; a silica layer on asurface of the molded polymer; an adherent bonded to the silica layer;and an article bonded to the adherent.
 17. The apparatus of claim 16,wherein the molded polymer comprises an overmold, the substratecomprises an electronic component, the silica comprises plasma convertedsilicone oil, and the article comprises a heat sink.
 18. The apparatusof claim 16, wherein the adherent comprises a silicone-based pasteadhesive with a metal oxide filler.
 19. The apparatus of claim 16,wherein the adherent comprises a porous polymer film impregnated withadhesive.
 20. The apparatus of claim 19, wherein the film comprisespolytetrafluoroethylene, the adhesive comprises polybutadine, and thefilm is further impregnated with a metal oxide heat transfer medium. 21.An apparatus comprising: a molded polymer formed on a substrate;silicon-containing residue on a surface of the molded polymer; anadherent bonded to the surface; and an article bonded to the adherent.22. The apparatus of claim 21, wherein the molded polymer comprises anovermold, the substrate comprises an electronic component, and thearticle comprises a heat sink.
 23. The apparatus of claim 21, whereinthe adherent comprises a porous polymer film impregnated with adhesive.24. The apparatus of claim 23, wherein silicon-containing residuecomprises silicone oil, the film comprises polytetrafluoroethylene, theadhesive comprises polybutadine, and the film is further impregnatedwith a metal oxide heat transfer medium.